Abstract/Summary Alzheimer?s disease (AD) currently has no cure, nor do we understand the initiating disease mechanisms, particularly those causing memory loss. This aspect of the disease is potentially mediated through synaptic deficits. To date, detailed investigations into early pathogenic mechanisms contributing to synaptic pathophysiology have been lacking. In part, this may reflect technical challenges in measuring real-time signaling events within synapses, and difficulties in translating findings from model animal systems to human neurons. Here, we address these gaps and identify cellular and molecular signaling mechanisms of AD pathogenesis that contributes to synaptic impairments. The objective of this proposal is to detect early features of synaptic pathology in AD mouse models and human neurons, and identify targetable mechanisms that emerge early in the disease process. To this end, we have identified dysregulated Ca2+ signaling as a candidate mechanism, and our studies test assertion this by characterizing abnormal Ca2+ responses within synaptic compartments generated by evoked activity at the hippocampal Schafer collateral synapse, detailing structural deficits and functional consequences in pre- and postsynaptic compartments, characterizing short- term plasticity and synaptic transmission deficiencies and linking this to protein alterations mediating signaling abnormalities in two AD mouse models and in human neurons where feasible. The underlying premise is based on findings in human patients and AD mouse models demonstrating early changes in ryanodine receptor type 2 (RyR2) expression and marked increases in RyR-Ca2+ release within dendritic compartments such as spines. These events precede the amyloid deposition, neuronal loss, and cognitive impairments that define AD. As tightly regulated Ca2+ signals are essential to synaptic functionality, sustained Ca2+ dyshomeostasis stands to be a significant factor in AD-associated synaptic pathology and cognitive decline. Our central hypothesis is that there are prodromal deficits in synaptic structure and function which are caused by early dysregulation of intracellular Ca2+ signaling. Using whole cell and field potential electrophysiological approaches, 2-photon (2P) and ratiometric imaging Ca2+ imaging, electron microscopy (EM), immunoassays and protein biochemistry approaches in AD mouse models, and in human neurons (iN) derived from AD patients, we will examine how dysregulated signaling manifests and drives synaptic pathology in AD. Aim I investigates the emergence and nature of early synaptic transmission and plasticity encoding deficits in AD mice and human iN. Aim II identifies structural abnormalities within and between synaptic compartments in AD brains and iN, and explores the protein modifications driving RyR dysregulation in spines. Aim III will determine if normalizing intracellular Ca2+ signaling preserves synaptic structure and function using siRNA and pharmacological approaches. The public health significance is that this study will identify a proximal source of synaptic degeneration in AD which directly links to processes causing memory and cognitive loss.